On amazon.co.uk the Kenko 2x Teleplus PRO 300 DGX is £144.99 while the Canon Extender EF 2x III is a staggering £399.

Ironically, there are very few reviews on both, but the Kenko does seem to have a slight "points" advantage, though that may be due to the lower price.

Before the 1D X firmware update providing f/8 AF, Art Morris blogged about getting AF at f/8 with the Kenko 1.4x on the 800/5.6L. At that time, I had just received my 600/4L II, and really wanted to know if the Kenko 2x would allow f/8 AF with an f/4 lens. I only found one comparison between the Kenko and Canon 2x TCs, on a birding forum. The guy had shot pics of a US$5 bill with both TCs behind a 500 II - with the Kenko, the bill was very soft, while the Canon 2x was sharp. Still, it was only one review. I had planned to test the Kenko 2x myself, and in fact, had both the Kenko 2x and 1.4x (for AF with the 100-400) in my B&H cart...but before I got around to clicking Buy, the firmware update was announced, so I bought the Canon 2xIII instead.